Because of improved interfaces to exploit the poor, a spammer can already pay to have humans solve captchas using an API, just as well as they could pay for computing time to solve hashes.
As such, if you were to tune the difficulty of a computing proof of work to be more expensive to compute than to pay the lowest bidding human farm to solve a captcha, it should be better at decreasing spam.
Because of improved interfaces to exploit the poor, a spammer can already pay to have humans solve captchas using an API, just as well as they could pay for computing time to solve hashes.
As such, if you were to tune the difficulty of a computing proof of work to be more expensive to compute than to pay the lowest bidding human farm to solve a captcha, it should be better at decreasing spam.